Silicon fusion bonding is used commercially to join together two or more silicon wafers, one or more of which may be oxidized.
The details of silicon fusion bonding are well published in the literature.
Silicon fusion bonding uses temperature and pressure to join atomically flat silicon wafers, one or more of which may be oxidized. General requirements for direct wafer fusion bonding include cleanliness and surface chemistry. General requirements for direct wafer fusion bonding also include microscopic smoothness with, and macroscopic flatness with RMS roughness of at most a few Ångströms and significantly less than 1.0 nanometer.
Any two flat, highly polished, clean surfaces will stick together if they are brought into contact. The bond is of the Van der Waal's, or hydrogen type and is of low strength, but can be significantly improved by thermal treatment. This silicon fusion bonding process has been successfully exploited for MEMS fabrication, using silicon-to-silicon fusion bonding with either plain or oxidized wafers.
Compared with anodic bonding, silicon fusion bonding results in exact thermal expansion matching that minimizes stress in bonded wafers; fusion bonded wafers have higher temperature capability whereas anodic bonding is limited by strain point of the glass; and fusion bonded wafers can be used for subsequent IC processing, whereas the anodic bonding process introduces alkali metal ions that cannot be allowed in CMOS processing.
However, surface roughness is a limiting factor in silicon fusion bonding. Surface roughness requirements for silicon fusion bonding is significantly less than 1.0 nanometer, being on the order of only few Ångströms. Fusion bonding of silicon or silicon dioxide requires that both surfaces be highly polished and smooth. If an oxide is deposited on the wafers, surface roughness may be too great to permit effective bonding. Thermal oxidation of the wafers typically results in lower surface roughness.
Wafer flatness or flexibility also limits bonding. If the wafers are warped, then the amount of surface area in close enough proximity is not sufficient to bond. Bowing in normal thickness wafers must be very small, as well as the surface roughness. Thinner wafers tend to deform easier and can overcome bow that thick wafers will not bond to normally.
Another limiting factor is surface chemistry. It is generally accepted that for silicon fusion bonding, hydrogen bonds dominate the strength or degree of bonding. Therefore, cleaning wafers in Piranha clean or another oxidizing bath immediately prior to bonding generally improves bonding.
Methods for fusion bonding of silicon are generally well-known. State-of-the-art silicon fusion bonding processes are disclosed, by example, in each of U.S. Pat. No. 5,286,671, “Fusion bonding technique for use in fabricating semiconductor devices;” U.S. Pat. No. 5,273,205; “Method and apparatus for silicon fusion bonding of silicon substrates using wet oxygen atmosphere;” and U.S. Pat. No. 6,629,465, “Miniature gauge pressure sensor using silicon fusion bonding and back etching,” which are all incorporated herein by reference. U.S. Pat. No. 5,286,671, for example, teaches a method for fabricating silicon-on-insulator (SOI) wafers by fusion bonding two silicon wafers.
Silicon fusion bonding is used in manufacture of Micro-Electro Mechanical System (MEMS) devices, of which monolithic silicon based sensors and actuators are subsets. MEMS silicon based sensor and actuator devices are generally well-known.
FIG. 1 is a simplified illustration of a generic MEMS device 1 of the prior art having one or more movable parts, such as a movable pendulum. As illustrated, the MEMS device 1 includes a silicon mechanism structure 2 sandwiched between two silicon substrates 3, 4 that form respective covers 5, 6 over the mechanism structure 2. The mechanism structure 2 may be formed of an epitaxial layer of silicon grown on one of the two silicon substrates 3, 4 that form the cover 5, 6, for example the silicon substrate 3 that form the lower cover 5. The mechanism structure 2 often includes one or more movable parts 7 such as a movable pendulum that is flexibly suspended by one or more flexures 8 from a relatively stationary frame 9 portion of the mechanism structure 2. The silicon mechanism structure 2 is often fusion bonded to the second of the two silicon substrates 3, 4, shown as the silicon substrate 4 that forms the upper cover 6.
Silicon fusion bond joints B are often formed between one or both of the substrates 3, 4 and respective opposing surfaces 10, 11 of the mechanism structure 2 to form the covers 5, 6 of the MEMS device 1. Accordingly, respective surfaces 12, 13 of the substrates 3, 4 to be silicon fusion bonded are machined flat and highly polished to a mirror smooth finish of only few Ångströms that is suitable for silicon fusion bonding, as is known in the art.
As illustrated, one or both of the two silicon substrates 3, 4 that form the covers 5, 6 is formed with a respective depression 14, 15 that is recessed into a respective surface 12, 13 of the substrates 3, 4. The depressions 14, 15 are positioned to provide clearance for the flexibly suspended movable pendulum or other movable part 7 to move either in-plane or out-of-plane with respect to the relatively stationary frame 9 portion of the mechanism structure 2. The depressions 14, 15 are formed in a controlled manner to a depth D of a few microns to as little as a fraction of a micron to provided the necessary clearance.
According to state-of-the-art fabrication methods known in the prior art, the one or more depressions 14, 15 are typically formed by a slow and well-controlled etch process that permits precise control of the depression depths D. Such precise depth control is obtained by either isotropically or anistropically etching in a suitable etchant, such as potassium hydroxide (KOH) for anistropically etching. Wet etching in KOH is a slow and well-controlled process that permits very precise control of the depth D of the depressions 14, 15. The slow and well-controlled KOH etch process also results in a mirror smooth finish of only few Ångströms on respective floor surfaces 16, 17 of the depressions 14, 15.
FIG. 2 illustrates that the mirror smooth finish of floor surfaces 16, 17 of the depressions 14, 15 tends to be a drawback when the mechanism structure 2 is formed with one or more of the movable parts 7. For some applications requiring a high degree of sensitivity, the one or more flexures 8 are sufficiently compliant as to permit the movable part 7 to droop or sag toward one of the covers 5, 6 when the device is not powered. During fabrication, at the operation of fusion bonding the silicon mechanism structure 2 with one or both of the cover substrates 3, 4, droop or sag of the movable part 7 may permit one surface 18, 19 thereof to contact the opposing floor surface 16, 17 of the corresponding depression 14, 15 in the respective cover substrates 3, 4.
One drawback of the mirror smooth finish of floor surfaces 16, 17 of the depressions 14, 15 is that very shallow depressions 14, 15 having a depth D of only a fraction of a micron may permit sufficient contact area between one of the movable part surfaces 18, 19 and the floor surface 16, 17 of the corresponding depression 14, 15 that silicon fusion bonding occurs therebetween during fusion bonding of the mirror finished mechanism surfaces 10, 11 to the respective surfaces 12, 13 of the substrates 3, 4. Thus, a partial or even full silicon fusion bond joint F is developed between the movable part 7 and one of the covers 5, 6 as a result of the mirror smooth finish of floor surfaces 16, 17 of the depressions 14, 15, whereby the movable part 7 is rendered immovable.